std::deque<T,Allocator>::shrink_to_fit

void shrink_to_fit();		(since C++11)

Requests the removal of unused capacity.

It is a non-binding request to reduce the memory usage without changing the size of the sequence. It depends on the implementation whether the request is fulfilled.

All iterators and references are invalidated. end() iterator is also invalidated.

Parameters

(none)

Type requirements
- `T` must meet the requirements of MoveInsertable.

Return value

(none)

Complexity

At most linear in the size of the container.

Notes

If an exception is thrown other than by T's move constructor, there are no effects.

Example

Run this code

#include <deque>
#include <iostream>
#include <new>
 
// minimal C++11 allocator with debug output
template <class Tp>
struct NAlloc
{
    typedef Tp value_type;
 
    NAlloc() = default;
 
    template <class T> NAlloc(const NAlloc<T>&) {}
 
    Tp* allocate(std::size_t n)
    {
        n *= sizeof(Tp);
        std::cout << "allocating " << n << " bytes\n";
        return static_cast<Tp*>(::operator new(n));
    }
 
    void deallocate(Tp* p, std::size_t n)
    {
        std::cout << "deallocating " << n*sizeof*p << " bytes\n";
        ::operator delete(p);
    }
};
template <class T, class U>
bool operator==(const NAlloc<T>&, const NAlloc<U>&) { return true; }
template <class T, class U>
bool operator!=(const NAlloc<T>&, const NAlloc<U>&) { return false; }
 
int main()
{
    // std::queue has no capacity() function (like std::vector).
    // Because of this, we use a custom allocator to show the
    // working of shrink_to_fit.
 
    std::cout << "Default-construct deque:\n";
    std::deque<int, NAlloc<int>> deq;
 
    std::cout << "\nAdd 300 elements:\n";
    for (int i = 1000; i < 1300; ++i)
        deq.push_back(i);
 
    std::cout << "\nPop 100 elements:\n";
    for (int i = 0; i < 100; ++i)
        deq.pop_front();
 
    std::cout << "\nRun shrink_to_fit:\n";
    deq.shrink_to_fit();
 
    std::cout << "\nDestroy deque as it goes out of scope:\n";
}

Possible output:

Default-construct deque:
allocating 64 bytes
allocating 512 bytes
 
Add 300 elements:
allocating 512 bytes
allocating 512 bytes
 
Pop 100 elements:
 
Run shrink_to_fit:
allocating 64 bytes
allocating 512 bytes
allocating 512 bytes
deallocating 512 bytes
deallocating 512 bytes
deallocating 512 bytes
deallocating 64 bytes
 
Destroy deque as it goes out of scope:
deallocating 512 bytes
deallocating 512 bytes
deallocating 64 bytes

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Technical specifications
Symbols index
External libraries

Sequence
array (C++11)
vector
vector<bool>
deque
forward_list (C++11)
list
Associative
set
multiset
map
multimap
Unordered associative
unordered_set (C++11)
unordered_multiset (C++11)
unordered_map (C++11)
unordered_multimap (C++11)
Adaptors
stack
queue
priority_queue
flat_set (C++23)
flat_multiset (C++23)
flat_map (C++23)
flat_multimap (C++23)
Views
span (C++20)
mdspan (C++23)
Iterator invalidation
Member function table
Non-member function table

Member types
Member functions
deque::deque
deque::~deque
deque::operator=
deque::assign
deque::assign_range (C++23)
deque::get_allocator
Element access
deque::at
deque::operator[]
deque::front
deque::back
Iterators
deque::begindeque::cbegin (C++11)
deque::enddeque::cend (C++11)
deque::rbegindeque::crbegin (C++11)
deque::renddeque::crend (C++11)
Capacity
deque::empty
deque::size
deque::max_size
deque::shrink_to_fit (DR*)
Modifiers
deque::clear
deque::insert
deque::insert_range (C++23)
deque::emplace
deque::erase
deque::push_front
deque::emplace_front (C++11)
deque::prepend_range (C++23)
deque::pop_front
deque::push_back
deque::emplace_back (C++11)
deque::append_range (C++23)
deque::pop_back
deque::resize
deque::swap
Non-member functions
std::swap
eraseerase_if (C++20)(C++20)
operator==operator!=operator<operator>operator<=operator>=operator<=> (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)
Deduction guides(C++17)